On Sun, May 12, 2019 at 11:32:28AM +0200, Ingo Molnar wrote: > > * Alexey Dobriyan <adobri...@gmail.com> wrote: > > > On Sat, May 11, 2019 at 11:02:24PM +0300, Alexey Dobriyan wrote: > > > I compiled current F29 kernel config on x86_64 (5.0.13-200.fc29.x86_64) > > > with -ffreestanding. The results are interesting :^): > > > > > > add/remove: 30/22 grow/shrink: 1290/46867 up/down: 33658/-1778055 > > > (-1744397) > > > Total: Before=83298859, After=81554462, chg -2.09% (!) > > > > > > That's original config with modules compiled built-in. > > > > Argh, it's the other way: adding -ffreestanding shrinks kernel by 2%. > > This is a very interesting finding, as we've seen numerous code > generation artifacts from GCC assuming libgcc things. > > Has anyone investigated by any chance where the -ffreestanding space > savings come from mostly - is it mostly in cold paths, or does it make or > hot codepaths more efficient as well? > > If it's the latter then the kernel would be directly faster as well > (fewer instructions executed), not just indirectly from better cache > packing, I suppse?
Turns out -ffreestanding completely disables stack protector :-\ F29 83298859 -ffreestanding -1744397 -2.09% STACKPROTECTOR_STRONG=n -1369949 -1.64% Builtin function are in noise, e.g -fno-builtin-sprintf is only -14KB.
